1. Field of the Invention
The present invention relates to a pulp and paper process and apparatus, and more particularly, to a new method and apparatus for drying paper.
2. Description of the Prior Art
Current industrial practice is to dry paper by passing the wet paper web over a series of cylinders containing saturated steam, in an atmosphere of air. This process is characterized by a relatively low drying rate, particularly towards the dry end, and by a high energy consumption. In many mills, the dryer section limits production capacity.
In recent years, there has been proposed the drying of paper by using superheated steam. Such a proposal is described in Oscar Luthi U.S. Pat. No. 4,242,808, issued Jan. 6, 1981. In the Luthi patent, a paper web is dried by directly contacting the paper web with superheated steam to evaporate the water in the paper web by direct contact and thus cooling the superheated steam. The exhaust steam is then again superheated and the superheated steam is again used in the drying of the paper web. The Luthi method is a completely sealed and closed circuit where air is completely eliminated from the drying system. According to Luthi, the heat content per unit volume of steam is approximately 25% greater than the heat content of air. In an enclosed drying process as described by Luthi, the water content in the paper is converted to steam upon impingement of the superheated steam and is exhausted with the superheated steam. This provides additional steam in the system which is in excess of the steam required for the closed circuit even given provisions for leakage, etc. As suggested by Luthi, the excess steam can be bled off to be used in other parts of the mill.